Labs / Brian's Brain

Brian's Brain

Take Life, add a third state, and forbid survival. Every cell that fires must spend a generation dying before its square can be used again. The result is a world in which almost nothing stands still.

Click and drag to fire cells by hand — doing so pauses the simulation so your drawing survives, and the Play button sets it running again. Drag over a firing cell to erase instead.

The rules

Each cell is in one of three states, and the transition is unconditional for two of them:

  1. A firing cell always becomes dying.
  2. A dying cell always becomes off.
  3. An off cell fires if and only if exactly two of its eight neighbours are firing.

In Generations notation this is written B2/S/3 - birth on two, no survival conditions at all, three states. It was devised by Brian Silverman, who also created WireWorld.

Why it behaves so differently

The dying state is a refractory period, borrowed directly from neurons: after a neuron fires it physically cannot fire again for a short interval. That single constraint changes everything. In Life, a stable object can sit in one place because its cells keep each other alive. In Brian's Brain that is impossible, because nothing survives - the only way for a structure to persist is for it to keep moving.

So the Brain has no still lifes and very few oscillators. What it has, in enormous quantity, is spaceships. A random soup collapses almost immediately into a sky full of gliders travelling in every direction, colliding, annihilating and occasionally producing new ones. The dying cells trail behind the firing ones, which is why everything appears to have a comet's tail.

The refractory period also prevents the wave of activity from reflecting backwards - a firing region cannot re-ignite the tissue it just came through. This is exactly the mechanism that makes waves in real excitable media travel in one direction, and it is why cellular automata of this shape are used to model cardiac tissue and the spiral waves that appear during fibrillation.

Things to try

  • Clear the grid and draw a short diagonal line of three or four cells - it will usually launch a spaceship.
  • Draw a long straight line and watch it break up symmetrically from both ends.
  • Turn the density right down for a new soup: sparse starts produce isolated ships you can actually follow.
  • Turn the density right up: dense starts produce a brief flash of activity that dies almost entirely, because too many neighbours fire at once and the "exactly two" condition fails everywhere.